The invention relates to a method and device for controlling the NOx regeneration of a NOx storage catalyst located in the exhaust gas channel of an internal combustion engine of a motor vehicle and having the features recited in the preambles of the independent claims.
It is known to integrate an exhaust gas cleaning device in the exhaust gas channel for the purpose of cleaning the exhaust of internal combustion engines. The exhaust gas cleaning device typically includes components such as a particle filters or catalysts. If a raw emission of NOx of the internal combustion engine is to be reduced, then these catalysts include a reduction catalyst. If the mass flows of reducing pollutants, such as carbon monoxide CO and incompletely combusted carbohydrides CH are sufficiently large in the region of the reduction catalyst, then the reducing agent NOx promotes a conversion to nitrogen.
To minimize fuel consumption, it has proven to be advantageous to operate the internal combustion engine under the lean air conditions. However, the operation in the range optimized for fuel consumption is associated, on one hand, with increased NOx emission and, on the other hand, with reduced mass flows of reducing agents. To prevent high NOx emission levels, a NOx storage device is associated with the catalyst which absorbs the NOx in form of a nitrate. The NOx storage device can be combined with the catalyst as a so-called NOx storage catalyst.
The mass storage capacity of the NOx storage catalyst is, of course, limited, so that a NOx regeneration has be performed in regular intervals to prevent NOx breakthroughs. During the NOx regeneration, the operating mode changes to stoichiometric or rich. The NOx that had been absorbed in the form of nitrate is thereby released. Typically, a NOx regeneration is initiated when a threshold value for a load state of the NOx storage catalyst or a NOx emission detected downstream by a NOx-sensitive measuring device (breakthrough emission) is exceeded. This has the disadvantage that identical criteria are applied for all operating phases of the motor vehicle to determine the need for regeneration. However, if the NOx regeneration is initiated in the idle phase, significantly less exhaust gas flows and therefore the mass flows of reducing agents that are present are also smaller, so that the desorbed NOx can only be incompletely reduced on the catalyst component. NOx regeneration during the idle phase does not only result in an undesirably high NOx emission, but also leads to increased fuel consumption as compared to other operating phases where the internal combustion engine runs under higher load. Disadvantageous, the NOx regeneration in the idle phase is frequently also accompanied by generation of undesirable noise. Moreover, NOx regeneration under idle conditions takes longer due to the smaller exhaust gas flows, and the operating conditions with unfavorable fuel consumption must be maintained for a longer time.